Selected Problems from
A First Course in Probability by Sheldon Ross

Problem 57 (Page 194)

pta = 
      1 - (E^(-λ)) - λ (E^(-λ)) - (E^(-λ)) ((λ^2)/2) /. {λ -> 3} // 
        N

ptb1 = 1 - (E^(-λ)) /. {λ -> 3} // N

(pta/ptb1)